Various methods are employed to measure very small currents (e.g.; pico amps to nano amps). Such methods generally employ a current source (current limited by high effective internal resistance) that has large offset potentials (e.g.; .+-. a few thousand volts, either polarity). The output of the current source is coupled to a sensitive current measuring device, such as an electrometer operating in current mode. A problem is that an instrument such as an electrometer typically only has voltage offset capabilities of a few hundred volts, and must therefore be floated to a high voltage in order to be used with the current source.
One example of a current source employed in measuring small currents is an electron multiplier. Another example of a current source employed in measuring very small currents is a Faraday cup. Applicants' invention has application in embodiments including various types of current sources. Electron multipliers will be described by way of example only.
An electron multiplier is an apparatus comprising a tube in which current amplification is realized through secondary emission of electrons. Secondary emission of electrons occurs when the surface of a material is bombarded by high velocity primary ions. The energy of incident primary ions is usually sufficient to liberate several secondary ions per incident particle. The bombarded surface is called a secondary emitter. The electron multiplier comprises a tube. The electron multiplier further comprises, in the tube, a cathode (or first voltage application electrode), a collector (or second voltage application electrode) spaced apart from the input cathode, and an electron multiplication region in the tube between the cathode and collector. There are two general types of electron multipliers: discrete dynode multipliers, and continuous dynode multipliers.
In discrete dynode electron multipliers, the electron multiplication region is defined by a plurality of discrete dynodes (anodes). The anodes are located in the tube between the cathode and the collector, on alternating sides of the tube. The anodes are made of a material which makes a good secondary emitter. A very high voltage is applied to the collector. A lower voltage is applied to the anode closest to the output collector. The voltage applied to the anode closest to the output collector is higher than a voltage applied to the anode which is second closest to the output collector, which is higher than a voltage applied to the anode which is third closest to the output collector, etc. In operation, electrons are accelerated through the tube by potential differences from one location of the tube to the next. For example, electrons are accelerated by the potential applied to the anode closest to the cathode (first anode), which is a high potential. When the electrons impact the first anode, a greater number of electrons is produced because the anodes are good secondary emitters. These electrons are accelerated by the next anode, which is at a higher potential than the previous anode, and by each subsequent anode, which are at increasingly higher potentials. A large output pulse is produced at the collector.
Continuous dynode multipliers operate on a similar principle, but do not include separate, discrete anodes. Instead, a tube of lead silicate glass is processed to exhibit electrical conductivity and secondary emission properties. The processed lead silicate glass defines a semiconducting layer. A first voltage is applied to the semiconducting layer at one end of the tube, and a second voltage is applied to the semiconducting layer at the other end of the tube. An example of a continuous dynode multiplier is a 4000 Series Channeltron (.TM.) electron multiplier manufactured by Galileo Electro-Optics Corporation (previously manufactured by the Electro-Optics Division of Bendix Corporation).
Electron multipliers can be operated in either an analog mode, or a pulse counting mode. Most are operated in analog mode. The difference between electron multipliers operating in pulse counting mode and electron multiplier operating in analog mode is that in pulse counting mode output pulses are produced with a characteristic output, whereas electron multipliers operating in analog mode have a very wide distribution of output pulse amplitudes that generally overlap due to the higher counting rates of analog multipliers.
Electron multipliers require that the exit end be biased much more positive (e.g., 1500-5000 volts more positive) than the entrance or cathode.
Electron multipliers, such as Galileo electron multipliers, are employed in measuring ions. When it is desired to measure negative ions, a sensitive current measuring device, such as an electrometer in current mode is connected to the collector of an electron multiplier. An electrometer is a device that measures potential difference or electric charge by sensing mechanical forces that exist between bodies that possess electrostatic charges. In order to be able to connect the electrometer to the output of the electron multiplier (without having to float the electrometer at a potential above ground), the collector of the electron multiplier is held generally at ground, and a very negative voltage is applied to the cathode. Because the voltage at the cathode is negative, an external conversion dynode is required at the entrance of the electron multiplier to convert negative ions to positive ions, and a very high positive voltage is applied to the dynode. Negative ions impact this dynode, and kick off positive secondary ions into the electron multiplier. The positive secondary ions are attracted to the electron multiplier and produce secondary electrons on impact. The sensitivity of the dynode method depends on the efficiency of positive ion production.
It is desirable to measure negative ions for various reasons. For example, it is useful to measure negative ions in mass spectrometry. Mass spectrometry, and the use of electron multipliers, is discussed in detail in chapter 18 of "Principles of Instrumental Analysis", Third Edition, Douglas A. Skoog, Saunders College Publishing, 1985.
Mass spectrometry is used, for example, to determine the structure of a molecule. In mass spectrometry, molecules of a sample are broken up into constituent parts (fragments) by collision with streams of electrons, ions, fast atoms, or photons (alternatively, fragmentation can be achieved thermally, or by applying a high electrical potential). Some of the resulting fragments are negative ions and some are positive ions. Either the positive or the negative ions are removed (e.g., by drawing the positive or negative ions through a slit in a mass analyzer, described below, using a large positive or negative potential). Each kind of ion has a particular mass to charge ratio (m/e ratio). Most ions have a charge of 1, and the mass to charge ratio is therefore simply the mass of the ion.
A mass analyzer receives the positive or negative ions and disperses them based upon the mass of the ions. Ions of a given mass are supplied to an electron multiplier.
The electron multiplier is used with the mass analyzer so that a signal representative of the relative abundance of each ion is produced. The intensity at the output of the electron multiplier indicates the abundance of an ion introduced into the electron multiplier. A plot or list of the intensities of each mass to charge ratio can be produced, and that plot or list is referred to as a mass spectrum.
A mass spectrum is highly characteristic of a particular compound. The mass spectrum can be used to assist in determining the structure of an unknown molecule, or to determine whether two molecules are identical to one another.
It should be noted that there are various types of mass analyzers, such as magnetic sector analyzers (single focusing or double focusing), quadrupole analyzers, and time of flight analyzers. The invention has application with any type of mass analyzer.
In a sector analyzer, a permanent magnet or electromagnet is used to cause an ion beam to be deflected into a circular path in an analyzer tube which is under vacuum and which has a slitted outlet leading to an electron multiplier. At the inlet of the tube is an ionization chamber including first and second spaced apart slitted walls which ions must pass through in sequence to reach the analyzer tube. Different mass particles can be selected for focusing on the outlet slit by varying the field strength of the magnet or the accelerating potential between the first and second slitted walls.
In a quadrupole analyzer, an ion source creates a beam of ionized particles. A quadrupole analyzer employs four short, parallel metal rods arranged symmetrically around the beam of ionized particles. Opposed rods are electrically connected such that one pair of rods is attached to the positive side of a variable DC source, and the other pair of rods is attached to the negative side of the variable DC source. Variable radio frequency AC signals, 180.degree. out of phase, are also applied to each pair of rods. Neither the DC nor the AC accelerates particles ejected from the ion source. The combined field effect, however, causes the particles to oscillate about their respective central axis of travel, and only those with a given range of mass to charge ratios can pass through the array without being removed by colliding into one of the rods. Bass scanning is achieved by varying the frequency of the ac supply while holding the potentials constant or by varying the potentials of both the AC and DC sources while keeping their ratio and the frequency constant.
In a time of flight analyzer, ions are produced intermittently by bombardment with pulses. The produced ions are accelerated by an electrical field pulse that has the same frequency as the ionization pulse, but which lags behind the ionization pulse. The accelerated particles pass into a field free drift tube which leads to the electron multiplier. Because all particles entering the drift tube have the same kinetic energy, their velocities in the drift tube varies inversely with their respective masses. Lighter particles arrive at the electron multiplier earlier than the heavier ones. The electron multiplier is used to determine the relative intensity of the various ions, and time of travel in the drift tube is used to determine the relative mass of various ions.
It is also useful to measure negative ions in another type of mass spectrometer, called an ion trap mass spectrometer. Ion trap mass spectrometers generally include trapped ion analyzer cells. Gaseous sample molecules are ionized in the center analyzer cell by electrons that are accelerated from a filament to a collector. A pulsed voltage is applied to a grid at the filament to switch the beam on and off periodically. Ions formed while the beam is on are trapped within the cell for a few seconds. The ions are held in place by an electrostatic well created by applying AC voltages to end caps and a ring electrode. The ions are accelerated out of the cell and into an electron multiplier which is connected to a preamplifier which amplifies the current.
Ion detection and mass spectrometry are discussed in U.S. Pat. No. 3,774,028 issued to Daly on Nov. 20, 1973; U.S. Pat. No. 3,898,456 issued to Dietz on Aug. 5, 1975; U.S. Pat. No. 4,267,448, issued May 12, 1981 to Feser et al.; U.S. Pat. No. 4,423,324 issued to Stafford on Dec. 27, 1983; and U.S. Pat. No. 4,808,818, issued to Jung on Feb. 28, 1989, all of which are incorporated herein by reference.
It is useful to measure negative ions in various other applications, such as in SIMS. A SIMS (Secondary Ion Mass Spectroscopy) system employs an ion beam (ion microprobe) to sputter material, in the form of secondary ions, from the surface of a sample such as a semiconductor, to detect impurities in the surface of the sample. The secondary ions are electrostatically accelerated and analyzed using a mass spectrometer as described above. Most of the secondary ions are emitted from the two top atomic layers of the sample. A depth profile of a sample can be obtained, in a destructive analysis technique, by sputtering the sample continuously in a vertical direction. Accuracy decreases, however, as depth increases.